A fatty ester of sucrose finds wide application in the fields of food, cosmetics, pharmaceuticals, chemical industry, etc. for many purposes such as emulsification, solubilization, dispersion, antibacterial treatment and inhibition of crystal growth. By properly selecting the substitution degree and kind of fatty acid constituting the fatty ester of sucrose, a fatty ester of sucrose having a wide range of HLB (hydrophilic-lipophilic balance) values can be prepared. Furthermore, a fatty ester of sucrose having a low HLB value can be dissolved in an organic solvent while one having a high HLB value can be dissolved in water or an alcohol. Therefore, these fatty esters of sucrose can be appropriately selected depending on the intended application. In the food industry, for example, a fatty ester of sucrose having a high HLB value is used in an amount of from 400 to 1,000 ppm for antibacterial effect in drinks such as coffee and black tea with dairy ingredient. A fatty ester of sucrose having a low HLB value is used in an amount of from 0.2 to 2% by weight for inhibiting the crystal growth of fats and oils in chocolate, etc.
In general, however, if a fatty ester of sucrose is used in the form of an aqueous solution, the greater the HLB or concentration of the fatty ester of sucrose as a solute, the higher the viscosity of the resulting aqueous solution. Therefore, the preparation of such an aqueous solution of a fatty ester of sucrose requires a large amount of energy and considerable time. Furthermore, it can be difficult to handle such an aqueous solution of a fatty ester of sucrose in various operations. For example, it is difficult to transfer a viscous solution of a fatty ester of sucrose. Accordingly, when used for the foregoing purposes, a dilute aqueous solution of a fatty ester of sucrose having a low viscosity is generally selected.
On the other hand, if whipped cream is prepared from raw cream, 45% of an oil component and 55% of an aqueous component (serum including sugar, protein, etc.) are used as raw materials to produce a product having an oil content of about 40%. In other words, the maximum amount of additional water which can be used is 12.5 parts by weight based on 100 parts by weight of the raw materials. Therefore, if a fatty ester of sucrose is to be added in an amount of about 1% based on the total weight of the system, it is necessary to prepare an aqueous solution of a fatty ester of sucrose having a concentration of about 8%.
Furthermore, a fatty ester of sucrose contains a hydroxyl group as a hydrophilic group and thus does not have the disadvantages of a poly(oxyethylene)derivatives emulsifying agent that loses emulsifying ability at high temperature due to the presence of cloud point. Therefore, the fatty ester of sucrose is advantageous in that the resulting emulsion has high heat stability. A fatty ester of sucrose, also, exhibits high biodegradability and a low stimulating action on the skin, thus it suits for cosmetics and medicines for external application.
In the field of cosmetic materials and pharmaceuticals, if an oil-in-water type emulsion having a high oil phase content such as a skin cosmetic [cream: JP-B-59-14019 (The term "JP-B" as used herein means an "examined Japanese patent publication"), Takeo Mitsui, "New Cosmetic Chemistry", page 349, May 10, 1994, Nanzando] or a liquid hair cream (JP-B-60-31802) is produced, the water content is limited. Therefore, it is necessary to use a highly concentrated aqueous solution of an emulsifying agent as a dispersing emulsifying agent. Accordingly, an aqueous solution of an emulsifying agent containing a fatty ester of sucrose having a concentration of not less than 8% by weight has been desired.
Also if a fatty ester of sucrose is stored in the form of a highly concentrated aqueous solution, problems arise due to high viscosity. Thus, hitherto, a fatty ester of sucrose had to be stored as a dilute aqueous solution having a concentration, for example, of not more than about 5%.